The present invention relates to a drive wheel slippage control device for an automotive vehicle, and more particularly relates to such a drive wheel slippage control device, which controls the braking system of an automotive vehicle during vehicle acceleration in order to prevent acceleration slippage of the drive wheels thereof.
In the prior art there is a known type of drive wheel slippage control device for an automotive vehicle, which detects when, during acceleration of the vehicle, one or more of the vehicle drive wheels is slipping, i.e. is spinning substantially faster than appropriate for the current value of vehicle speed, and which in such circumstances operates a brake device on said slipping drive wheel so as to curb said slippage thereof. In principle, this is effective for preventing excessive tire wear and for enhancing safe vehicle operation, but some problems arise with such a simple form of prior art vehicle drive wheel slippage control.
Namely, in such a prior art vehicle drive wheel slippage control device, regardless of the degree of brake pedal depression, hydraulic pressure or the like is provided for operating the braking system for one or more of the vehicle wheels, so as to stop the excessive spinning of the one of said vehicle drive wheels which is currently slipping. Thus, the acceleration slippage control response may be good. For example, referring to the synchronized graphs of FIG. 2 in which time is shown along the horizontal axis and drive wheel rotational speed and also brake actuating hydraulic fluid pressure are shown along the vertical axes, it may be practiced to decide that acceleration slippage is occurring when the rotational speed Vr of the drive wheel is greater than a determinate rotational speed Vs which is based upon the vehicle road speed Vf, or alternatively when the acceleration of the rotation of said drive wheel is greater than some determinate value; in either case, it is conventional to then operate the braking system for said drive wheel uniformly regardless of the actual currently prevailing road surface conditions, so as to prevent the further slippage of said drive wheel. However, this ignoring of the current road surface conditions, i.e. of the coefficient of friction between the vehicle drive wheels and the road surface, means that satisfactory slippage control is difficult or virtually impossible to obtain, and the problem of reduced vehicle accelerability occurs.
Specifically, when the road surface is dry and has no moisture or ice upon it, then the coefficient of friction between the vehicle drive wheels and the road surface is relatively high, and vehicle drive wheel slippage is difficult. If in these circumstances drive wheel slippage nevertheless does occur, then it is likely that a relatively high braking force on the relevant drive wheel will be needed to control such slippage, and that this relatively high braking force will be needed to be applied for a relatively long time period before drive wheel slippage is fully curbed. On the other hand, when the road surface is wet or icy and has moisture or ice upon it, then the coefficient of friction between the vehicle drive wheels and the road surface is relatively low, and vehicle drive wheel slippage is relatively easy to occur. If in these circumstances drive wheel slippage does in fact occur, then it is likely that only a relatively low braking force on the relevant drive wheel will be needed to control such slippage, and that this relatively low braking force will only be needed to be applied for a relatively short time period before drive wheel slippage is fully curbed. However, since in the conventional art the braking force applied to the slipping drive wheel is substantially uniform regardless of road surface conditions, and since such braking force, when vehicle drive wheel slippage occurs, is applied fo a substantially uniform time period again regardless of road surface conditions, thereby the dilemma arises that: if such braking force and the time for its application are adjusted so as to correspond to a road surface of relatively high coefficient of friction (relative to the vehicle drive wheels), then in the case of a road surface of relatively low such coefficient of friction the amount and the time of braking of a slipping vehicle drive wheel will be excessive, resulting in bad vehicle accelerability; while, on the other hand, if such braking force and the time for its application are adjusted so as to correspond to a road surface of relatively low coefficient of friction (relative to the vehicle drive wheels), then in the case of a road surface of relatively high such coefficient of friction the amount and the time of braking of a slipping vehicle drive wheel will be too low, resulting in bad curbing of vehicle road wheel slippage and the elapsing of a long time period before a slippage episode is finally prevented. Thus, the conventional art is unsatisfactory.